This invention relates to an illumination optical system and an exposure apparatus using the same. More particularly, the invention is suitably applicable to an illumination system in an exposure apparatus for use in a micro-lithography process in the manufacture of various devices having a very fine pattern structure, such as semiconductor devices, liquid crystal devices, or magnetic devices, for example.
In exposure apparatuses used for semiconductor device manufacture, a higher and higher resolution has been required to meet further increases in density of each semiconductor device. A super-resolution technology using oblique incidence illumination or a phase shift mask is known, in this respect. In such super-resolution technology, an aperture stop of an illumination optical system is changed to make smaller, the sigma value (the ratio between the numerical aperture (NA) of a projection optical system and the numerical aperture (NA) of an illumination optical system), or a secondary light source of a particular shape such as a ring-like shape or quadruple shape, for example, is produced.
Further, exposure apparatuses for semiconductor device manufacture generally include an optical system which is telecentric on the image side, such that there occurs substantially no positional deviation of an image even if a photosensitive substrate such as a wafer is at a defocused position.
However, with recent increases in density of VLSIs, an extraordinarily high transfer precision is required in the photolithographic exposure of a circuit pattern, and it is very important to correctly set an angular distribution of exposure light incident on a photosensitive substrate. If the incidence angle of the exposure light incident on the photosensitive substrate is not proper, and when the photosensitive substrate is at a defocused position, a positional deviation of an image occurs. Any deviation in incidence angle of the exposure light with respect to the photosensitive substrate, produced with respect to the magnification and about the optical axis center (i.e., an error in the magnification telecentricity), if the photosensitive substrate is defocused, causes a positional deviation, in a printed pattern, of the magnification component corresponding to the image height.
This means that, during plural lithographic processes, an image shift occurs when a circuit pattern is printed superposedly upon a photosensitive substrate having a surface step formed thereon, and it is very unadvisable with respect to the manufacture of high precision semiconductor devices.
In many exposure apparatuses, positions of components of an illumination optical system are so adjusted that, in a certain standard illumination mode A, exposure light of an optimum angular characteristic and illuminance distribution can be supplied. However, when the illumination mode is changed from illumination mode A to another illumination mode B such as oblique incidence illumination or small-sigma illumination, for example, if the components of the illumination optical system are held at the same positions as in the illumination mode A, the angular characteristic of the exposure light cannot always be made optimum and, also, the illuminance distribution changes. This is mainly because the light path is different for different illumination modes, and the influence of an anti-reflection film applied to an optical element changes with the angle of light rays.
It is accordingly an object of the present invention to provide an exposure apparatus by which high illuminance uniformness and correct angular characteristic can be assured in every illumination mode of exposure light.
In accordance with an aspect of the present invention, there is provided an illumination optical system characterized by means for changing an illuminance distribution and an angular distribution of illumination light, independently of each other.
In accordance with another aspect of the present invention, there is provided an illumination optical system including an optical integrator for forming a secondary light source by use of light from a light source, and a condenser optical system for condensing light from said optical integrator upon a surface to be illuminated, characterized in that said condenser optical system is arranged to change, independently of each other, an illuminance distribution and an angular distribution of the light incident on the surface illuminated.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.